A typical can-annular gas turbine engine 10 such as manufactured by the assignee of the present invention is illustrated in partial cross-sectional view in FIG. 1. The engine 10 includes a plurality of combustors 12 (only one illustrated) arranged in an annular array about a rotatable shaft 14. The combustors 12 receive a combustible fuel from a fuel supply 16 and compressed air from a compressor 20 that is driven by the shaft 14. The fuel is combusted in the compressed air within the combustors 12 to produce hot combustion gas 22. The combustion gas 22 is expanded through a turbine 24 to produce work for driving the shaft 14. The shaft 14 may also be connected to an electrical generator (not illustrated) for producing electricity.
The hot combustion gas 22 is conveyed from the combustors 12 to the turbine 24 by a respective plurality of transition ducts 26. The transition ducts 26 each have a generally cylindrical shape at an inlet end 28 corresponding to the shape of the combustor 12. The transition ducts 26 each have a generally rectangular shape at an outlet end 30 corresponding to a respective arc-length of an inlet to the turbine 24. The plane of the inlet end 28 and the plane of the outlet end 30 are typically disposed at an angle relative to each other. The degree of curvature of the radially opposed sides of the generally rectangular outlet end 30 depends upon the number of transition ducts 26 used in the engine 10. For example, in a Model 501 gas turbine engine supplied by the assignee of the present invention, there are sixteen combustors 12 and transition ducts 26, thus each transition duct outlet end 30 extends across a 22.5° arc of the turbine inlet. A Model 251 engine supplied by the present assignee utilizes only eight combustors 12 and transition ducts 26, thus each transition duct outlet end 30 extends across approximately a 45° arc.
The high firing temperatures generated in a gas turbine engine combined with the complex geometry of the transition duct 26 can lead to a temperature-limiting level of stress within the transition duct 26. Materials capable of withstanding extended high temperature operation are used to manufacture transition ducts 26, and ceramic thermal barrier coatings may be applied to the base material to provide additional protection. Active cooling of the transition duct 26 with either air or steam may be used. Steam cooling is provided by routing steam from an external source through internal cooling passages formed in the transition duct 26. Air cooling may be provided by utilizing the compressed air flowing past the transition duct 26 between the compressor and the combustor or from another source. Cooling air may be routed through cooling passages formed in the transition duct 26, or it may be impinged onto the outside (cooled) surface of the transition duct 26, or it may be allowed to pass through holes from the outside of the transition duct 26 to the inside provide a barrier layer of cooler air between the combustion air and the duct wall (effusion cooling). Further details regarding such cooling schemes may be found in U.S. Pat. No. 5,906,093, which describes a method of converting a steam-cooled transition duct to air-cooling, and United States patent application publication US 2003/0106317 A1, which describes an effusion cooled transition duct. Both of these documents are hereby incorporated by reference in their entirety.